Optical wavelength cross connect architectures using wavelength routing elements

ABSTRACT

An optical wavelength cross connect is provided to receive multiple input optical signals that each have multiple spectral bands and to transmit multiple output optical signals that each have one or more of those spectral bands. The optical wavelength cross connect includes multiple wavelength routing elements, which are optical components that selectively route wavelength components between one optical signal and multiple optical signals in either direction according to a configurable state.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part application of U.S. patent application Ser. No. 10/093,844, entitled “OPTICAL WAVELENGTH CROSS CONNECT ARCHITECTURES USING WAVELENGTH ROUTING ELEMENTS,” filed Mar. 8, 2002 by Edward J. Bortolini el al., the entire disclosure of which is herein incorporated by reference for all purposes. This application is also related to the following commonly assigned applications, the entire disclosure of each of which is also herein incorporated by reference for all purposes: U.S. patent application Ser. No. 10/093,843 entitled “METHODS FOR PERFORMING IN-SERVICE UPGRADES OF OPTICAL WAVELENGTH CROSS CONNECTS,” filed Mar. 8, 2002 by Edward J. Bortolini; U.S. patent application Ser. No. 10/126,189, entitled “MULTI-CITY DWDM WAVELENGTH LINK ARCHITECTURES AND METHODS FOR UPGRADING,” filed Apr. 19, 2002 by S. Christopher Alaimo et al.; and U.S. patent application Ser. No. 10/150,810, entitled “BIDIRECTIONAL WAVELENGTH CROSS-CONNECT ARCHITECTURES USING WAVELENGTH ROUTING ELEMENTS,” filed May 17, 2002 by Edward J. Bortolini et al.

BACKGROUND OF THE INVENTION

[0002] This application relates generally to fiber-optic communications. This application relates more specifically to optical wavelength cross-connect architectures used in fiber-optics applications.

[0003] The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data.

[0004] The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.

[0005] In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronics—typically an electronic SONET/SDH system. However, multi-wavelength systems generally require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology. The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called “wavelength routing networks” or “optical transport networks” (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable.

[0006] Optical wavelength cross connects are configured generally to redirect the individual optical channels on a plurality of input optical fibers to a plurality of output optical fibers. Each incoming channel may be directed to any of the output optical fibers depending on a state of the cross connect. Thus, where there are P input fibers and Q output fibers, the optical wavelength cross connect between them may be considered to be a “PN×QN optical switch.” Sometimes herein, the terminology “P×Q optical wavelength cross connect” is used to refer to such a cross connect by referring to the numbers of input and output optical fibers, each of which is understood to have the capacity for carrying N channels. As such the “P×Q optical wavelength cross connect” terminology may be considered to be a shorthand for describing a arbitrarily configurable PN×QN optical device.

[0007]FIG. 1 provides an example of a prior-art 4×4 optical wavelength cross connect 100 for a DWDM system carrying N individual wavelength channels. Each of the N channels on the four input signals 104 may be redistributed in accordance with a state of the cross connect 100 among the four output signals 116. The cross connect 100 functions by splitting each of the input signals 104(i) with an optical demultiplexer 108(i) into N signals 120(1 . . . N, i) that carry only a single wavelength channel λ_(1 . . . N). From each of the optical demultiplexers 108, the signal corresponding to a particular one of the 120(j, 1 . . . 4) is directed to a respective one of N 4×4 optical space switches 110(j). Each optical space switch 110 may be configured as desired to redirect the four received signals 120 to four transmitted signals 124. The transmitted signals 124 are transmitted to optical multiplexers 112 that recombine the reordered individual-wavelength signals onto the four output signals 116.

[0008] The efficiency of an arrangement such as shown in FIG. 1 is limited because it adopts a brute-force-type approach of demultiplexing the four incoming signals into their individual 4N components in order to reroute them. There is a general need in the art for more efficient optical wavelength cross-connect architectures without compromising complete routing flexibility.

BRIEF SUMMARY OF THE INVENTION

[0009] Embodiments of the invention thus provide optical wavelength cross-connect architectures having such routing flexibility. In one set of embodiments, an optical wavelength cross connect is provided that receives a plurality of input optical signals that each have a plurality of spectral bands and transmits a plurality of output optical signals that each have one or more of the spectral bands. The optical wavelength cross connect comprises a first plurality of wavelength routing elements. Each of the wavelength routing elements is an optical component adapted for selectively routing wavelength components between a first optical signal and a plurality of second optical signals according to a configurable state. In general, a wavelength routing element may be configured to receive the first optical signal and route the spectral bands to the plurality of second optical signals or may be configured to receive the plurality of second optical signals and route the spectral bands to the first optical signal. As used within the optical wavelength cross connect, each of the first plurality of wavelength routing elements is disposed to receive at least one optical signal corresponding to one of the input optical signals. A mapping of the spectral bands comprised by the plurality of input optical signals to the plurality of output optical signals is determined by the states of the first plurality of wavelength routing elements. The wavelength routing elements may generally be configured in any fashion, and may include, for example, four-pass or two-pass wavelength routing elements.

[0010] The optical wavelength cross connect may include both working and protection fabrics, with at least one of the first plurality of wavelength routing elements comprised by the protection fabric and the remainder comprised by the working fabric. A plurality of working optical switches are configured to transmit the output optical signals by selecting either a corresponding working signal from the working fabric or a protection signal from the protection fabric. The protection fabric may also include an optical switch for selecting to which of the working optical switches any protection signal should be directed. Either or both of the working and protection fabrics may also include optical amplifiers disposed to amplify the working and/or protection signal prior to transmission to the working optical switches.

[0011] The signals received by the wavelength routing elements in the cross connect will generally be either the input signals directly or will be equivalents of the input signals resulting from splitting the input optical signals with optical splitters. Such optical splitters may also be disposed to direct equivalents of the input optical signals to the protection fabric. In some embodiments, the optical splitters provide equivalents of every input signal to every one of the first plurality of wavelength routing elements. The working and protection signals are thus defined directly by the respective states of the wavelength routing elements. In another embodiment, the optical splitters provide an equivalent of only one of the input optical signals to each of the wavelength routing elements on the working fabric but provide equivalents of all the input optical signals to the wavelength routing element(s) on the protection fabric. In such an embodiment, a second plurality of wavelength routing elements is disposed to receive each of the outputs from the first plurality of working-fabric wavelength routing elements. The outputs from the second plurality of wavelength routing elements thus correspond to the output optical signals.

[0012] The cross connect may also be equipped with additional optical equipment in alternative embodiments. For example, in one embodiment, a plurality of optical performance monitors are disposed to measure optical characteristics of various signals, such as of the input optical signals, the output optical signals, or any of the signals propagated through different points of the working and/or protection fabrics. In another embodiment, optical supervisory channels may be disposed to drop spectral bands from the input optical signals or to add spectral bands to the output optical signals. Such optical supervisory drops and adds may be useful where the wavelength routing elements are configured according to a predetermined wavelength grid and the dropped or added spectral bands to not correspond to position on the predetermined grid.

[0013] In other embodiments, channel-blocking capabilities of wavelength routing elements may be exploited in forming optical wavelength cross connects and in providing methods for distributing spectral bands from input optical signals onto output optical signals. In any of these embodiments, the input optical signals may be equal in number to the output optical signals, although this is not a requirement. In one embodiment, the spectral bands from each of the input optical signals are selectively distributed onto a plurality of intermediate optical signals. The intermediate optical signals are duplicated, with spectral bands being selected from the duplicated optical signals to provide the output optical signals. In selecting spectral bands from the duplicated optical signals, propagation of certain spectral bands may be blocked, such as by using an optical-channel-blocking embodiment of a wavelength routing element.

[0014] In another embodiment, spectral bands from each of the input signals are selectively distributed onto a plurality of intermediate optical signals, with propagation of certain spectral bands being blocked in such a distribution. Spectral bands from combinations of the intermediate optical signals are selected to provide the output optical signals.

[0015] In another embodiment, a first plurality of intermediate optical signals are produced that are equivalent to corresponding input optical signals. A second plurality of intermediate optical signals are also produced by selectively distributing spectral bands from each of the plurality of input optical signals. Combinations of the second plurality of intermediate optical signals are combined to provide a portion of the output optical signals. Spectral bands are selected from each of the first plurality of intermediate optical signals to provide a remainder of the output optical signals. The selection of spectral bands from each of the first plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands from the duplicated signals onto the remainder of the output optical signals. Similarly, producing the second plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands onto the second plurality of intermediate optical signals.

[0016] In yet another embodiment, a first plurality of intermediate optical signals is produced equivalent to a portion of the plurality of input signals. A second plurality of intermediate optical signals is produced by selectively distributing spectral bands from each of a remainder of the plurality of input optical signals. Combinations of the second plurality of intermediate optical signals are combined with selected spectral bands from the first plurality of intermediate optical signals to produce the output optical signals. In some instances, producing the second plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands onto the second plurality of intermediate optical signals. Similarly, in some instances, selecting spectral bands from the first plurality of intermediate optical signals may comprise blocking propagation of certain spectral bands from the first plurality of intermediate optical signals.

[0017] In still further embodiments, arrays of smaller cross connects configured using wavelength routing elements may be arranged to provide larger cross-connect functions. For example, a K×K optical wavelength cross connect may be configured to distribute spectral bands comprised by K input optical signals among K output optical signals as an array of interconnected optical wavelength cross connects, at least one of which is smaller than K×K. The mapping of the spectral bands from the input ports to the output ports is determined by states of the interconnected optical wavelength cross connects. Examples of array configurations that may be used include crossbar arrays, Benes arrays, Benes arrays with spatial dilation, and Clos arrays. In one embodiment, a generalized Clos array is used in which the array comprises three layers. The first layer has a plurality m of k×n optical wavelength cross connects, the second layer has a plurality k of m×m optical wavelength cross connects, and the third layer has a plurality m of n×k optical wavelength cross connects. Each optical wavelength cross connect in the second layer receives an intermediate input signal from each of the optical wavelength cross connects included in the first layer and transmits an intermediate optical signal to each of the optical wavelength cross connects included in the third layer. In one embodiment, k=2n−1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings wherein like reference numerals are used throughout the several drawings to refer to similar components. In some instances, a sublabel is associated with a reference numeral and is enclosed in parentheses to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sublabel, it is intended to refer to all such multiple similar components.

[0019]FIG. 1 is a schematic diagram illustrating a prior-art cross connect used in DWDM applications;

[0020]FIGS. 2A, 2B, and 2C are schematic top, side, and end views, respectively, of an optical wavelength routing element used in certain embodiments of the invention;

[0021]FIGS. 3A and 3B are schematic top and side views, respectively, of an optical wavelength routing element used in certain embodiments of the invention;

[0022]FIG. 4 is a schematic top view of an optical routing element according to a third embodiment of the invention;

[0023]FIGS. 5A-5D are schematic diagrams showing examples of P×1 cross-connect building blocks that include wavelength routing elements in accordance with embodiments of the invention;

[0024]FIGS. 6A and 6B are schematic diagrams showing examples of 2×3 and 3×2 cross-connect building blocks that include wavelength routing elements in accordance with embodiments of the invention;

[0025]FIGS. 6C, 6D, and 6E schematically summarize various categories of P×1 cross-connect building blocks that include wavelength routing elements in accordance with embodiments of the invention;

[0026]FIGS. 7A and 7B are schematic diagrams showing optical wavelength cross-connect architectures using a broadcast-and-select arrangement according to embodiments of the invention;

[0027]FIG. 8A is a flow diagram showing a method for performing an in-service upgrade of an optical wavelength cross-connect in accordance with an embodiment of the invention;

[0028]FIGS. 8B-8G are schematic diagrams illustrating the configuration of an optical wavelength cross-connect at different stages of an in-service upgrade performed in accordance with the flow diagram of FIG. 8A;

[0029]FIGS. 9A and 9B are schematic diagrams showing optical wavelength cross-connect architectures using a distribute-and-select arrangement according to embodiments of the invention;

[0030]FIG. 10 is a schematic diagram showing an optical wavelength cross-connect architecture in another embodiment of the invention;

[0031]FIG. 11 is a schematic diagram showing an optical wavelength cross-connect architecture in a further embodiment of the invention; and

[0032]FIGS. 12A-12E are schematic diagrams illustrating alternative cross-connect architectures according to further embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] 1. Introduction

[0034] The following description sets forth embodiments of optical wavelength cross-connect architectures according to the invention. The general operation of such cross-connect architectures is to receive P input signals at respective input ports and output Q output signals at respective output ports. Each of the input and output signals comprises a plurality of spectral bands, with the cross connect capable of achieving a configuration that results in a desired redistribution of input spectral bands corresponding to equivalent channels among the output signals. Although the signals could each have a continuous spectrum, adjacent segments of which could be considered different spectral bands, it is generally contemplated that the spectrum of the incoming light will have a plurality of spaced bands, denoted as corresponding to channels 1, 2, 3, . . . N. In some instances, the examples provided herein focus on symmetric cross connects in which P=Q, but this is not a requirement and embodiments of the invention may readily be adapted to nonsymmetric cross connects also.

[0035] The terms “input port” and “output port” are intended to have broad meanings. At the broadest, a port is defined by a point where light enters or leaves the system. For example, the input (or output) port could be the location of a light source (or detector) or the location of the downstream end of an input fiber (or the upstream end of an output fiber). In specific embodiments, the structure at the port location could include a fiber connector to receive the fiber, or could include the end of a fiber pigtail, the other end of which is connected to outside components. The optical character of the system also permits the input ports and output ports to be interchanged functionally, permitting, for example, a P×Q element to be used as a Q×P element.

[0036] The International Telecommunications Union (ITU) has defined a standard wavelength grid having a frequency band centered at 194,100 GHz, and another band at every 50 GHz interval around 194,100 GHz. This corresponds to a wavelength spacing of approximately 0.4 nm around a center wavelength of approximately 1550 nm, it being understood that the grid is uniform in frequency and only approximately uniform in wavelength. Embodiments of the invention are preferably designed for the ITU grid, but finer frequency intervals of 25 GHz and 100 GHz (corresponding to wavelength spacings of approximately 0.2 nm and 0.8 nm) are also of interest.

[0037] 2. Wavelength Routing Element

[0038] Embodiments of the invention for an optical wavelength cross connect include one or more wavelength routing elements (“WRE”). As used herein, a “1×L WRE” refers to an optical device that receives multiplexed light at a WRE input port and redirects subsets of the spectral bands comprised by the multiplexed light to respective ones of a plurality L of WRE output ports. Such a 1×L WRE may be operated as an L×1 WRE by interchanging the functions of the input and output ports. Specifically, a plurality L of optical signals, each multiplexed according to the same wavelength grid are provided at the L output ports (functioning as input ports). A single optical signal is output at the input port (functioning as an output port) and includes spectral bands selected from the L multiplexed optical signals according to the wavelength grid. Thus, the single output optical signal has, at each position on the wavelength grid, no more than one spectral band received at the same position on the wavelength grid from the L multiplexed optical signals. Accordingly, reference herein to a WRE adapted for routing wavelength components “between” a first optical signal and a plurality of second optical signals is intended to include a WRE configured to operate as a 1×L WRE or a WRE configured to operate as an L×1 WRE.

[0039] Embodiments for the cross connects that use a WRE may generally use any configuration for routing subsets of a plurality of spectral bands that achieve these functions. In some instances, a particular WRE may be provided in a one-pass, two-pass, four-pass, or other configuration. Some examples of suitable WREs are described in detail below, and additional examples of WREs that may be comprised by certain embodiments are described in the copending, commonly assigned U.S. patent application, filed Nov. 16, 1999 and assigned Ser. No. 09/442,061 (“the '061 application”), entitled “Wavelength Router,” by Robert T. Weverka et al., which is herein incorporated by reference in its entirety, including the Appendix, for all purposes.

[0040] In some embodiments, wavelength routing functions within the WRE may be performed optically with a free-space optical train disposed between the WRE input port and the WRE output ports, and a routing mechanism. The free-space optical train can include air-spaced elements or can be of generally monolithic construction. The optical train includes a dispersive element such as a diffraction grating. The routing mechanism includes one or more routing elements and cooperates with the other elements in the optical train to provide optical paths that couple desired subsets of the spectral bands to desired WRE output ports. The routing elements are disposed to intercept the different spectral bands after they have been spatially separated by their first encounter with the dispersive element.

[0041]FIGS. 2A, 2B, and 2C are schematic top, side, and end views, respectively, of one embodiment of a 1×L (or, equivalently, L×1) WRE 210. This embodiment may be considered to be a four-pass WRE. Its general functionality is to accept light having a plurality N of spectral bands at a WRE input port 212, and to direct subsets of the spectral bands to desired ones of a plurality L of WRE output ports, designated 215(1) . . . 215(L). The output ports are shown in the end view of FIG. 2C as disposed along a line 217 that extends generally perpendicular to the top view of FIG. 2A. Light entering the WRE 10 from WRE input port 212 forms a diverging beam 218, which includes the different spectral bands. Beam 218 encounters a lens 220 that collimates the light and directs it to a reflective diffraction grating 225. The grating 225 disperses the light so that collimated beams at different wavelengths are directed at different angles back towards the lens 220.

[0042] Two such beams are shown explicitly and denoted 226 and 226′, the latter drawn in dashed lines. Since these collimated beams encounter the lens 220 at different angles, they are focused towards different points along a line 227 in a transverse plane extending in the plane of the top view of FIG. 2A. The focused beams encounter respective ones of a plurality of retroreflectors, designated 230(1) . . . 230(N), located near the transverse plane. Various examples of micromirror configurations that may be used as part of the retroreflectors, among others, are described in the following copending, commonly assigned applications, each of which is herein incorporated by reference in its entirety for all purposes: U.S. patent application Ser. No. 09/898,988, entitled “SYSTEMS AND METHODS FOR OVERCOMING STICTION USING A LEVER,” filed Jul. 3, 2001 by Bevan Staple et al.; U.S. patent application Ser. No. 09/899,000, entitled “FREE-SPACE OPTICAL WAVELENGTH ROUTER BASED ON STEPWISE CONTROLLED TILTING MIRRORS,” filed Jul. 3, 2001 by Victor Buzzetta et al.; U.S. patent application Ser. No. 09/899,001, entitled “TWO-DIMENSIONAL FREE-SPACE OPTICAL WAVELENGTH ROUTER BASED ON STEPWISE CONTROLLED TILTING MIRRORS,” filed Jul. 3, 2001 by Victor Buzzetta; U.S. patent application Ser. No. 09/899,002, entitled “MEMS-BASED, NONCONTACTING, FREE-SPACE OPTICAL SWITCH,” filed Jul. 3, 2001 by Bevan Staple and Richard Roth; U.S. patent application Ser. No. 09/899,004, entitled “BISTABLE MICROMIRROR WITH CONTACTLESS STOPS,” filed Jul. 3, 2001 by Lilac Muller; U.S. patent application Ser. No. 09/899,014, entitled “METHODS AND APPARATUS FOR PROVIDING A MULTI-STOP MICROMIRROR,” filed Jul. 3, 2001 by David Paul Anderson; and U.S. patent application Ser. No. 09/941,998, entitled “MULTIMIRROR STACK FOR VERTICAL INTEGRATION OF MEMS DEVICES IN TWO-POSITION RETROREFLECTORS,” filed Aug. 28, 2001 by Frederick Kent Copeland.

[0043] The beams are directed back, as diverging beams, to the lens 220 where they are collimated, and directed again to the grating 225. On the second encounter with the grating 225, the angular separation between the different beams is removed and they are directed back to the lens 220, which focuses them. The retroreflectors 230 may be configured to send their intercepted beams along a reverse path displaced along respective lines 235(1) . . . 235(N) that extend generally parallel to line 217 in the plane of the side view of FIG. 2B and the end view of FIG. 2C, thereby directing each beam to one or another of WRE output ports 215.

[0044] Another embodiment of a WRE, designated 210′, is illustrated with schematic top and side views in FIGS. 3A and 3B, respectively. This embodiment may be considered an unfolded version of the embodiment of FIGS. 2A-2C and operates as a two-pass WRE. Light entering the WRE 10′ from WRE input port 212 forms diverging beam 218, which includes the different spectral bands. Beam 218 encounters a first lens 220 a , which collimates the light and directs it to a transmissive grating 225′. The grating 225′ disperses the light so that collimated beams at different wavelengths encounter a second lens 220 b , which focuses the beams. The focused beams are reflected by respective ones of plurality of retroreflectors 230, which may also be configured as described above, as diverging beams, back to lens 220 b , which collimates them and directs them to grating 225′. On the second encounter, the grating 225′ removes the angular separation between the different beams, which are then focused in the plane of WRE output ports 215 by lens 220 a.

[0045] A third embodiment of a WRE, designated 210″, is illustrated with the schematic top view shown in FIG. 4. This embodiment is a further folded version of the embodiment of FIGS. 2A-2C, shown as a solid glass embodiment that uses a concave reflector 240 in place of lens 220 of FIGS. 2A-2C or lenses 220 a and 220 b of FIGS. 3A-3B. Light entering the WRE 210″ from input port 212 forms diverging beam 218, which includes the different spectral bands. Beam 218 encounters concave reflector 240, which collimates the light and directs it to reflective diffraction grating 225, where it is dispersed so that collimated beams at different wavelengths are directed at different angles back towards concave reflector 240. Two such beams are shown explicitly, one in solid lines and one in dashed lines. The beams then encounter retroreflectors 230 and proceed on a return path, encountering concave reflector 240, reflective grating 225′, and concave reflector 240, the final encounter with which focuses the beams to the desired WRE output ports. Again, the retroreflectors 230 may be configured as described above.

[0046] 3. Cross-Connect Building Blocks

[0047] Architectures for large cross connects made in accordance with certain embodiments of the invention use L×1 optical elements that include one or more WREs. Such an element is referred to generically herein as an “L×1 WRE,” including arrangements that have more than one WRE, provided at least one WRE is comprised by the element. Thus, one example of an embodiment of an L×1 WRE that may be used in cross-connect architectures according to the invention is a single structure that has one input (output) port and L output (input) ports. Other embodiments of an L×1 WRE comprised of smaller WREs are illustrated in FIGS. 5A-5D.

[0048] For example, FIG. 5A shows how a 4×1 WRE 510 may be configured with three 2×1 WREs. Each of the 2×1 WREs used in any of these embodiments may be configured as one of the WREs described in the '061 application or may be configured according to another WRE design. The 4×1 WRE 510 accepts four input signals 502 and outputs a single output signal 515. The four input signals 502 are received in pairs by two of the 2×1 WREs 504. The outputs from the 2×1 WREs 504 are used as inputs to the third 2×1 WRE, which output the output signal 515.

[0049] This arrangement of 2×1 WREs may thus be considered to be a tree arrangement. At each level of the tree, the number of distinct spectral bands across all optical signals at that level is reduced by the action of the 2×1 WREs 504 until, at the final level, only the desired spectral bands remain on the output signal 535. The resulting 4×1 WRE 510 thus functions according to the definition provided above for the operation of a WRE by mapping selected spectral bands from each of the input signals 502 according to a wavelength grid.

[0050] The embodiment of FIG. 5A may also be used as a 1×4 WRE to perform the reverse mapping according to the wavelength grid by interchanging the functions of the input and output ports. In such an instance, spectral bands originating on the single input signal are progressively directed to the desired ones of the plurality of output signals by separating them with the 2×1 WREs 504 at each level of the tree. It is thus evident for a 1×4 WRE (and more generally for a 1×L WRE) that certain wavelength-grid positions of at least some of the output signals will be inactive by carrying no spectral bands.

[0051]FIG. 5B shows an extension of the tree arrangement of 2×1 WREs 504 to an architecture that provides an 8×1 WRE 520. Spectral bands from eight input signals 522 are routed according to a unique wavelength-grid assignment to a single output signal 525. The eight input signals 522 are received in pairs by four 2×1 WREs 504, and the four outputs from those 2×1 WREs are received by the 4×1 WRE 510 shown in FIG. 5A. The resulting 8×1 WRE 520 functions according to the definition provided above for the operation of a WRE by mapping selected spectral bands from each of the input signals 522 according to a wavelength grid. It may also be used as a 1×8 WRE to perform the reverse mapping according to the wavelength grid by interchanging the functions of the input and output ports.

[0052] It is evident that larger WREs may be configured by including more layers in the tree. Adding still another layer of 2×1 WREs to the 8×1 WRE of FIG. 5B results in a 16×1 WRE. More generally, for a tree having p full layers of 2×1 WREs, the resulting element functions as a 2^(P)×1 WRE, mapping spectral bands from 2 ^(P) input signals according to a wavelength grid onto a single output port. Such an element may alternatively be used as a 1×2^(P) WRE to perform the reverse mapping according to the wavelength grid by interchanging the functions of the input and output ports.

[0053] It is not necessary that every level of the tree be completely filled with 2×1 WREs. For example, FIG. 5C provides a schematic illustration of an embodiment similar to that of FIG. 5B except that two of the 2×1 WREs 504 at the widest level of the tree have been removed. Accordingly, this embodiment functions as a 6×1 WRE 530 that maps selected spectral bands from each of six input signals 532 according to a wavelength grid onto a single output signal 535. Interchanging the functions of input and ports results in a reverse mapping according to the wavelength grid so that element 530 functions as a 1×6 WRE. It is noted by showing the component 4×1 WRE 510 with the dashed line that this embodiment may alternatively be considered as a configuration having a complete tree, but with different sizes of WREs on a given level. The 6×1 WRE 530 shown comprises a tree having a 4×1 WRE 510 and a 2×1 WRE 504 on its widest level, these WREs feeding into a 2×1 WRE 504 at the top level.

[0054] Similarly, FIG. 5D eliminates some 2×1 WREs 504 from two levels of the tree when compared with FIG. 5B. The illustrated embodiment functions as a 5×1 WRE 540 by mapping selected spectral bands from each of five input signals 542 according to a wavelength grid onto a single output signal 545. As for the other embodiments, element 540 may function as a 1×5 WRE by interchanging the functions of the input and output ports. Also, like the embodiment shown in FIG. 5C, element 540 may be considered as having WREs of different sizes, specifically in this example of comprising a 4×1 WRE 510 and a 2×1 WRE 504.

[0055] It is evident that various other combinations may be made according to the principles described with respect to FIGS. 5A-5D to produce L×1 and 1×L WREs for any value of L.

[0056]FIGS. 6A and 6B provide examples respectively of 2×3 and 3×2 cross-connect building blocks. The illustrated embodiments use combinations of 2×1 WREs, which may be configured as described in the '061 application or otherwise, and optical splitters. The embodiment shown in FIG. 6A functions as a 2×3 cross connect that maps spectral bands from two input optical signals 612 according to a wavelength grid onto three output signals 615. Each of the input optical signals 612 encounters a 1:3 optical splitter 608 connected with three 2×1 WREs 604. This arrangement thus provides a duplicate of both input signals 612 to each of the 2×1 WREs 604, each of which is configured to select the desired spectral bands for its corresponding output signal 615. Notably, this arrangement permits any combination of the spectral bands available from either of the input signals 612 to be included on any of the output signals 615, subject to the constraint imposed be the wavelength grid. Thus, for example, the specific spectral band at λ₀ on the wavelength grid for, say, the first input signals 612(1), may be included on one, two, or even all three of the output signals depending on the configuration of the 2×1 WREs 604. It is even possible for all of the output signals 615 to include an identical set of selected spectral bands from the two input signals 612.

[0057]FIG. 6B provides an example of a 3×2 cross-connect building block that operates on similar principles. Each of the three input signals 622 encounters a 1:2 optical splitter which directs duplicates of the input signals according to the arrangement illustrated in the figure. The routing of the duplicates with the illustrated set of 2×1 WREs permits each of the output signals 625 to include any desired combination of spectral bands from the input signals 622, subject to the wavelength-grid constraint and depending on the states of the 2×1 WREs. As for the arrangement shown in FIG. 6A for a 2×3 cross-connect building block, a specific spectral band from any of the input signals may be included on any (or all) of the output signals if desired.

[0058] The cascaded arrangements of WREs described with respect to FIGS. 5A-5D are themselves subsets of a more general classification of L×1 WREs that is summarized more comprehensively in FIGS. 6C-6E. The cascaded arrangements are configured so that the output of one component WRE is in optical communication with the input of another component WRE. In addition to such cascaded arrangements, the tabulation shown in FIGS. 6C-6E also provides examples of “flat WRE” embodiments and optical-channel-blocking (“OChB WRE”) embodiments. In addition to the other capabilities of WREs, the OChB WREs have the ability to selectively block a spectral band present on an input from appearing at an output. This may be achieved, for example, by using component WREs in which an “off” state is provided, such as by providing another state to the retroreflectors 230 identified in FIGS. 2A-4. For example, where such retroreflectors comprise tiltable mirrors, three states may be provided for a 1×2 OChB WRE by using mirrors that may be fully tilted to the left, fully tilted to the right, or in an intermediate untilted position. Several specific examples of such OChB WRE configurations are enabled by the WREs with off positions described in copending, commonly assigned U.S. patent application Ser. No. 10/099,392, entitled “ONE-TO-M WAVELENGTH ROUTING ELEMENT,” filed Mar. 13, 2002 by Nicholas C. Cizek et al., the entire disclosure of which is herein incorporated by reference for all purposes. For the flat and OChB embodiments shown in FIGS. 6C-6E that use a plurality of component WREs, no component WRE has its output in optical communication with the input of another component WRE. Furthermore, in the flat WRE embodiments that use a plurality of component WREs, each of the component WREs has an unused port while in the OChB embodiments that use a plurality of component WREs, at least one of the component WREs has all of its ports used. In particular OChB embodiments, the minimum number of WREs possible of a particular L are used. Such embodiments have advantages of reduced insertion loss, greater reliability, and more modest space requirements than do some other embodiments.

[0059] The tabulation in FIGS. 6C-6E illustrates certain examples of 3×1, 4×1, 6×1, and 8×1 WREs that may be made using component 1×2, 1×3, or 1×4 WREs. Such component WREs in this illustration are configured as single structures having one input (output) port and two, three, or four output (input) ports respectively. The three columns of FIG. 6C, denoted collectively by reference numeral 640(1), use component 1×2 WREs, with each of columns 642(1), 642(2), and 642(3) corresponding to cascaded, flat, and OChB configurations respectively. Similarly, the three columns of FIG. 6D, denoted collectively by reference numeral 640(2), use component 1×3 WREs, with each of columns 644(1), 644(2), and 644(3) corresponding to cascaded, flat, and OChB configurations respectively. Further arrangements using 1×4 WREs are shown in the three columns of FIG. 6E 640(3), also for the cascaded 646(1), flat 646(2), and OChB 646(3) configurations. The tabulation may clearly be extended both in terms of the size of the L×1 WREs and in terms of the component 1×M WREs for arbitrary M and L. Moreover, the tabulation shown is not exhaustive since other configurations that may be grouped according to the classifications are possible, even for those examples of specific L×1 WREs and component 1×M WREs already shown in FIGS. 6C-6E.

[0060] 4. K×K Optical Wavelength Cross Connects

[0061] a. Broadcast-and-Select Optical Wavelength Cross Connects

[0062] The building-block architectures illustrated in FIGS. 6A and 6B are examples of a more general class of cross-connect architectures described herein as “broadcast-and-select architectures.” A common feature of such architectures is that the input optical signals are duplicated with optical splitters, with a duplicate of each of the input signals being provided to a WRE, which may then be configured to select any of the desired spectral bands. There is therefore no constraint prohibiting a specific spectral band from any of the input signals from appearing on whatever number of output signals is desired. Such a capacity may be especially suitable for certain applications, including video applications among others.

[0063] 1. High-Reliability Embodiments

[0064] The broadcast-and-select architectures may be equipped with a protection capability. An example of such an architecture for a 4×4 cross connect 700 is illustrated in FIG. 7A, although it is evident how the principles may be used for a K×K cross connect of any size. In FIG. 7A, the cross connect comprises working fabric, denoted 704, and protection fabric, denoted 708. The working fabric includes a number K of K×1 WREs 712, each of which is configured to receive a duplicate of each of the input signals 702. In a manner similar to that described with respect to FIG. 6A, each of the K×1 WREs 712 may be configured to select whichever spectral bands are desired for the corresponding output signals 740.

[0065] The protection fabric 708 also includes a K×1 WRE 712′ configured to receive a duplicate of each of the input signals 702. In the event of a failure in the system affecting the output from one of the WREs 712 included on the working fabric 704, the protection WRE 712′ may be configured to substitute for the WRE 712 affected by the failure. Such substitution is accomplished with an arrangement of fiber switches. First, the protection fabric 708 comprises a 1×K fiber switch that receives the output of the protection WRE 712′ and directs it to one of K fiber switches provided as 2×1 fiber switches 716. Each of these 2×1 fiber switches 716 may select between a signal received from an associated WRE 712 on the working fabric and a signal from the 1×K fiber switch on the protection fabric, i.e. corresponding to a signal from the protection WRE.

[0066] Thus, in normal operation, each of the 2×1 fiber switches 716 is configured to transmit the optical signals received from their respective WREs 712 comprised by the working fabric 704 as output signals 740. In the event of a failure affecting one of the working WREs 712, transmission of the otherwise affected output signal 740 is preserved by using the protection fabric: the protection WRE 712′ is configured to reproduce the signal otherwise produced by the affected working WRE 712; the 1×4 fiber switch comprised by the working fabric 708 is configured to transmit that signal to the 2×1 fiber switch 716 corresponding to the affected working WRE 712; and that 2×1 fiber switch 716 is configured to transmit the signal from the protection path, i.e. from the 1×4 fiber switch 717, rather than the signal from the working path.

[0067] Duplicates of the input signals 702 are provided to all of the 4×1 WREs by 1:5 optical splitters 720 configured to encounter each of the input signals 702 and directing their outputs to the WREs. More generally, with a broadcast-and-select configuration for a K×K cross connect having a single protection WRE, the optical splitters used will be 1:(K+1) splitters. The strength of the duplicate signals will be reduced as a result of the power splitting performed by the splitters 720. Accordingly, optical amplifiers 714 and 714′ are provided on the working and protection fabrics 704 and 708 to strengthen the output signals 740. In the illustrated embodiment, the optical amplifiers 714 and 714′ are positioned respectively after the working and protection WREs 712 and 712′ along the optical paths followed by the optical signals, although other positions within the cross-connect architecture may alternatively be used. In one embodiment, the optical amplifiers 714 and 714′ comprise erbium-doped fiber amplifiers (“EDFAs”).

[0068] Further optional components of the cross connect 700 include a plurality of optical performance monitors (“OPMs”) 732. The OPMs act as low-power taps into the optical paths that may be used for monitoring the optical channels. Such OPMs 732 are shown in the illustrated embodiment positioned in three distinct positions along the optical paths: before the optical splitters 720; after the optical amplifiers 714 on both the working and protection fabrics 704 and 708; and after the 2×1 fiber switches 716. Each of these locations may be used for different monitoring purposes. The OPMs 732 positioned before the optical splitters 720 permit identification of signal faults before the active operation of the cross connect so that corrective action may be taken. Similarly, the OPMs 732 positioned after the 2×1 fiber switches 716 may be used to ensure no signal faults propagate along the output signals 740. Including OPMs 732 on the working and protection fabrics 704 and 708 provides signal monitoring that permits isolation of each of the WREs 712 and 712′ in diagnosing the source of any faults that may be detected. Such flexibility is particularly useful in some instances, including during the performance of an in-service upgrade as described below.

[0069] Optical supervisory channel (“OSC”) components may also optionally be included on the input and output signals. Such components are useful in instances where the input signal 702 includes out-of-band signals, i.e. includes additional spectral bands distinct from the wavelength grid used by the WREs. An OSC drop 724 may thus be included on each of the input signals 702 to detect and remove any such out-of-band signals. The out-of-band signals may then be added back to the output signals 740 with OSC adds 728 on the output signals.

[0070] 2. Ultrahigh-Reliability Embodiments

[0071] The reliability of the K×K cross connect may be increased by several orders of magnitude by including an additional WRE on the protection fabric. This is illustrated for an 8×8 cross connect 750 in FIG. 7B. The basic operation of the cross connect 750 is the same as the cross connect 700 described with respect to FIG. 7A. Working fabric 754 includes eight 8×1 WREs 762 configured to receive duplicates of input signals 752 from optical splitters 770. The optical splitters 770 are 1:10 splitters to accommodate the eight WREs 762 comprised be the working fabric and two WREs 762′ comprised by the protection fabric 758. Each of the signals output by the working WREs 762 encounters a 3×1 fiber switch 766 that receives signals from its associated working WRE 762 and may also receive signals from two 1×8 fiber switches 767 included on the protection fabric 758 and optically connected with each of the protection WREs 762′. The 3×1 fiber switches 766 transmit the appropriate signal as output signal 790. Amplifiers 764 and 764′ may be included as shown on the working and protection fabrics 754 and 758 to accommodate the power losses from the optical splitters 770, or may be positioned elsewhere in other embodiments. OPMs 782 may also be provided to monitor the performance of optical signals at different points in the cross-connect architecture, including on the working and protection fabrics 754 and 758 in some embodiments. OSC drops 774 and adds 778 may be provided for removing out-of-band signals from the input signals 752 and including them on the output signals 790.

[0072] Operation of the 8×8 cross connect shown in FIG. 7B is substantially the same as operation of the 4×4 cross connect shown in FIG. 7A. The large overall increase in reliability afforded by the two protection WREs 762′ may be understood by noting that there is statistically a mean time to failure for any element comprised by the cross connect. In the event of a failure, traffic is rerouted to one of the protection WREs 762′ for at a time period corresponding to a mean time to repair the failure. During that repair-time window,-there is a small but finite probability of a second failure, which may be protected by routing traffic to the second protection WRE 762′. It is even possible to accommodate the very remote possibility of certain types of third failures under some circumstances where the working path suffered only a partial failure. Traffic from the third failure, if consistent with the maintained operation of the partially failed path, may be rerouted to the partially failed path.

[0073] It is evident from the description of FIGS. 7A and 7B how to arrange a K×K cross-connect architecture having F protection paths. Each of K WREs included on the working fabric and F WREs included on the protection fabric comprises a K×1 WRE. A total of K optical splitters are thus provided as 1:(K+F) splitters. The fiber switches provided on the protection fabric are 1×K fiber switches and the fiber switches transmitting the output signals are (F+1)×1 fiber switches.

[0074] 3. In-Service Upgrades

[0075] The broadcast-and-select architectures described with respect to FIGS. 7A and 7B permit an existing K_(i) ₁ ×K_(j) ₁ architecture to be upgraded to a K_(i) ₁ ×K_(j) ₂ architecture without disrupting traffic using the cross connect. The illustration below focuses on the special case of symmetric cross connects where K_(i) ₁ =K_(j) ₁ and K_(i) ₂ =K_(j) ₂ , but such a limitation is not necessary and the upgrade may be performed more generally on asymmetric cross connects. The method for performing such an upgrade is shown schematically for the general case with the flow diagram of FIG. 8A and is shown at sequential stages in FIGS. 8B-8G for the specific case of upgrading a 2×2 cross connect (K_(i) ₁ =K_(j) ₁ =2) having a single protection path to a 4×4 cross connect (K_(i) ₂ =K_(j) ₂ =4). The following description thus refers simultaneously to the general method of FIG. 8A and the specific example in FIGS. 8B-8G. For convenience of description, the example of FIGS. 8B-8G omits OSC components, amplifiers, and OPMs, although it is understood that such components may be included in the architecture in different embodiments.

[0076] At block 801 of FIG. 8A, the method begins with a K_(i) ₁ ×K_(j) ₁ cross connect such as the 2×2 cross connect shown in FIG. 8B. The 2×2 cross connect includes two 2×1 WREs on the working fabric and a 2×1 WRE 814′ optically connected with a 1×2 fiber switch 818 on the protection fabric. Optical splitters 812 encounter the input signals 810 to direct duplicates to the WREs 814 and 814′. The output signals 820 are emitted from 2×1 fiber switches 816 configured to transmit working or protection signals depending on the configuration of the cross connect.

[0077] At block 802 of FIG. 8A, new working fabric is added to the cross-connect architecture to accommodate the new input and output ports. In upgrading the exemplary 2×2 cross connect to a 4×4 cross connect, the WREs on the working and protection fabrics will be replaced with 4×1 WREs. Thus in FIG. 8C, the cross connect is shown with two 4×1 WREs 824 added to the working fabric and optically connected with two new 2×1 fiber switches 816 that will be used to transmit new output signals.

[0078] At block 803 of FIG. 8A, the protection fabric is upgraded to the larger size. Thus, as shown in FIG. 8D for the exemplary upgrade to a 4×4 cross connect, the 2×1 WRE 814′ and 1×2 fiber switch 818 are replaced with a 4×1 WRE 824′ and a 1×4 fiber switch 828 respectively. Optical connections between the resulting protection fabric and other optical components of the system are completed. It is noted that such a four-channel protection fabric is capable of protecting traffic input from any number of input signals 810 up to four, including in particular the two input signals 810 being maintained during the upgrade. Also, while FIG. 8D illustrates simply upgrading the existing protection fabric to a larger size, in other embodiments the protection fabric is further upgraded by adding one or more additional protection paths with additional WREs and fiber switches, producing a protection fabric similar to that described with respect to FIG. 7B.

[0079] At this point in the method, each component of the working fabric is upgraded sequentially by transferring the traffic that uses that component of the working fabric to the protection fabric and performing the upgrade of that component. This is illustrated with a loop in the flow diagram of FIG. 8A, with traffic from the first component being transferred to the protection fabric at the first encounter of block 804. After upgrading the bypassed component of the working fabric at block 805, the cross connect appears as in FIG. 8E, with the first 2×1 WRE 814 substituted with a 4×1 WRE 824.

[0080] In addition, FIG. 8E illustrates how the operation of the optical splitters may be extended by adding 1:2 optical splitters as necessary at the outputs of the existing optical splitters. Such 1:2 optical splitters are added at points in the process when those outputs are not actively in use. Thus, in FIG. 8E 1:2 optical splitters 822 are added to the first outputs of the 1:3 splitters 812 while the traffic for the first input signal 810(1) is routed to the protection traffic. Optical connections are also made between the upgraded working WRE 824 and the (new) first outputs of the effectively expanded optical splitters. Optical splitters to be configured for encountering the new traffic to be added to the finally upgraded cross connect may be similarly configured as a combination of 1:3 and 1:2 splitters as shown. Alternatively, 1:5 splitters may be substituted. In the event that an additional protection channel is to be added, 1:6 splitters may be used instead to accommodate it.

[0081] After upgrading the bypassed component of the working fabric, the transmission of the bypassed traffic is switched back to the upgraded working fabric at block 806 of FIG. 8A. A determination is made at block 807 whether all of the working fabric has been upgraded. If not, the next component of the working fabric is upgraded at block 808 with the cycle repeating until the entire working fabric has been upgraded. Thus, FIG. 8F shows a further intermediate illustration of the cross connect when the traffic for the second input signal 810(2) is bypassed to the protection fabric. A similar upgrade is performed on the second component of the working fiber from a 2×1 WRE 814 to a 4×1 WRE 824 while the traffic is bypassed. Also, the 1:2 optical splitters 822 are added to the second outputs of the 1:3 optical splitters 812 and optically connected with the new 4×1 WRE 824.

[0082] After all the working fabric has been upgraded, additional optical signals 810(3) and 810(4) may be provided to, and additional output signals 820(3) and 820(4) emitted from, the cross connect, as shown in FIG. 8G. The cross connect now functions in every respect as a 4×4 broadcast-and-select cross-connect architecture. As shown in FIG. 8G, the third output of all the 1:3 optical splitters 812 is connected with a 1:2 optical splitter 822, resulting in an unused output. As alluded to above, the unused outputs may be optically connected with a second protection WRE on the protection fabric. Furthermore, for an architecture that uses only a single protection WRE, it is unnecessary to include the 1:2 optical splitters 822 on the third outputs of the 1:3 optical splitters 812 to include all optical connections with the working and protection fabrics; they are therefore omitted in an alternative embodiment. It is also noted that if the original K×K cross connect includes two protection paths, one of the protection paths may still function operationally as a protection path during the upgrade while the other protection path is used as a bypass while upgrading the working fabric.

[0083] It is noted that while FIGS. 8A and 8B-8G illustrate one embodiment for an in-service upgrade in which the protection fabric is upgraded before the working fabric is upgraded, such an order is not necessary. More generally, the upgrade of the protection fabric may more be performed at any point in the method, i.e. before, after, or even during the upgrade of the working fabric. With respect to the flow diagram of FIG. 8A, the position of block 803 may thus be freely moved within the method without exceeding the scope of the invention.

[0084] b. Distribute-and-Select Optical Wavelength Cross Connects

[0085] In alternative embodiments, the optical wavelength cross connect is configured as a distribute-and-select cross connect. Illustrations of a 4×4 cross connect configured in this way are provided in FIGS. 9A and 9B and correspond in functionality to the 4×4 broadcast-and-select cross connect shown schematically in FIG. 7A. To illustrate the basic operation of the distribute-and-select architecture, FIG. 9A provides an example of a configuration without protection. The working fabric in the distribute-and-select cross connect 900 includes a set of 1×4 WREs 910 and a set of 4×1 WREs 912. In the illustrated embodiment, each of the 1×4 WREs 910 receives one of the input signals 902.

[0086] The four outputs of each 1×4 WRE 910 are connected to an input port for each of the four 4×1 WREs 912. In normal operation, each of the 1×4 WREs 910 acts to distribute the spectral bands from its associated input signal 902 to the particular 4×1 WREs 912 that correspond to the desired output signal 940 for those spectral bands. Each of the 4×1 WREs 912 thus receives the spectral bands to be included on its associated output signal 940 and acts to multiplex those spectral bands accordingly.

[0087] This distribute-and-select architecture may be supplemented by additional features in a manner similar to that described with respect to the broadcast-and-select architecture. For example, amplifiers 914 may be included to compensate for loss of signal strength as optical signals are propagated through the system. In one embodiment the amplifiers 914 comprise erbium-doped fiber amplifiers. The amplifiers 914 may be positioned as shown in the embodiment of FIG. 9A, directly after the 4×1 WREs 912 along the optical paths followed in the cross connect, or may be positioned at other points in the system in other embodiments.

[0088] The cross connect 900 may also comprise OPMs 952 positioned at various points in the cross connect 900 to act as low-power taps for monitoring the optical channels. Such OPMs 952 are shown in the illustrated embodiment positioned in before each of the 1×4 WREs 910 and after the amplifiers 914 that follow the 4×1 WREs 912, although other positions may be used alternatively. The OPMs 952 permit isolated signal monitoring of the WREs in diagnosing the source of any faults that may be detected.

[0089] In addition, OSC drops 944 and adds 948 may be provided respectively on the input and output signals 902 and 940 for removing and adding out-of-band signals.

[0090] In some embodiments, the distribute-and-select architecture may include a protection fabric, such as illustrated in FIG. 9B. The operation of the protection-fabric embodiment shown in FIG. 9B may be explained more simply by labeling optical signals internal to the cross connect. This is done illustratively in FIG. 9A, in which internal optical signals emanating from the 1×4 WREs 910 are labeled Oαβ and internal optical signals received by the 4×1 WREs 912 are labeled Iβ′α′. Signals that are directed from the kth 1×4 WRE 910(k) to the kth 4×1 WRE 912(k) are unlabeled. For each of the other “O” signals emanating from the 1×4 WREs 910, α defines the originating 1×4 WRE 910(α) and β defines the receiving 4×1 WRE 912; because the labeling convention limits β to 1, 2, or 3, the same value of β may be used in different Oαβ labels to refer to different receiving 4×1 WREs 912. Similarly, for each of the other “I” signals received at the 4×1 WREs 912, α′ defines the originating 1×4 WRE 910 and β′ defines the receiving 4×1 WRE 912(β′); because the labeling convention limits α′ to 1, 2, or 3, the same value of α′ may be used in different Iβ′α′ labels to refer to different originating 1×4 WREs 910.

[0091] This labeling convention for internal signals is also used in FIG. 9B, in which the 4×4 distribute-and-select cross connect 900′ includes a protection fabric 908. The working fabric is denoted 904 and has a general functionality that corresponds to that described with respect to FIG. 9A. Rather than receive the input signals 902 directly, however, splitters 920 are disposed to intercept each of the input signals 902, sending an equivalent of each to both the working fabric 904 and the protection fabric 908. For input signal 902(k), the equivalent signal directed to the protection fabric is denoted OPk. Similarly, 2×1 fiber switches 916 are provided to select signals from either the working fabric 904 or the protection fabric 908 and to direct the selected signals as output signals 940. The signal received from the protection fabric 908 and corresponding to output signal 940(k) is denoted IPk.

[0092] Associated with each 1×4 WRE 910 on the working fabric 904 are 1×2 fiber switches 932 that may be configured to direct an optical signal received from the 1×4 WRE 910 either to elsewhere on the working fabric 904 or to the protection fabric 908. Similarly, associated with each 4×1 WRE 912 on the working fabric 904 are 2×1 fiber switches 934 that may be configured to direct an optical signal received either from elsewhere on the working fabric 904 or from the protection fabric 908 to the 4×1 WRE 912.

[0093] As shown in FIG. 9B, the protection fabric 908 includes a 1×4 WRE 910′ and a 4×1 WRE 912′ configured similarly to the corresponding elements comprised by the working fabric 904. A selection of the equivalent optical signals OPk directed to the 1×4 WRE 910′ is determined by a state of a 4×1 fiber switch 924 disposed to intercept the equivalent optical signals OPk. Similarly, a 1×4 fiber switch 927 is disposed to determine how to direct the signal propagated by the 4×1 WRE 912′ onto signals IPk. Associated with the 1'4 WRE 910′ are 1×4 fiber switches 926 that may be configured to redirect signals from the 1×4 WRE 910′ to different points in the working fabric 904. Associated with the 4×1 WRE 912′ are 4×1 fiber switches 928 that may be configured to direct signals from the working fabric to the 4×1 WRE 912′.

[0094] As in FIG. 9A, the internal optical signals on both the working and protection fabrics 904 and 908 are conventionally labeled Oαβ for signals emanating from the 1×4 WREs 910 and 910′, and Iβ′α′ for signals directed to the 4×1 WREs 912 and 912′. The same conventional notation is used as in FIG. 9A, with the WREs 910′ and 912′ on the protection fabric 908 being denoted as the fifth WREs so that O5β and I5α′ refer to optical signals internal to the protection fabric 908. The propagation of the internal optical signals to effect the operation of the cross connect is determined by the states of the 1×2 fiber switches 932, the 2×1 fiber switches 934, the 1×4 fiber switches 926, and the 4×1 fiber switches 928. For example, with the configuration of such switches as shown in FIG. 9B, the cross connect 900′ does not use the protection fabric and operates identically to the cross connect 900 as shown in FIG. 9A. For example, in both FIGS. 9A and 9B, from the first 1×4 WRE 910(1), signal O11 is directed to signal I21, signal O12 is directed to signal I31, signal O13 is directed to signal I41, and the unlabeled signal is directed directly to the 4×1 WRE 912(1). Similarly, in both FIGS. 9A and 9B, to the first 4×1 WRE 912(1), signal I11 is received from signal O21, signal I12 is received from signal O31, signal I13 is received from signal O41, and the unlabeled signal is received directly from the 1×4 WRE 910(1). It can be readily verified that all of the other internal working signals in the figures correspond.

[0095] In the event of a failure, traffic may be protected by changing the configuration of certain fiber switches and selecting the appropriate configurations for the protection WREs 910′ and 912′. For example, in the event of a failure of one of the first WREs 910(1) or 912(1), traffic that would use those WREs 910(1) and 912(1) may be rerouted to the protection fabric. This is done by: (1) setting the 1×4 fiber switch 924 to OP1 so that traffic otherwise directed to the first working 1×4 WRE 910(1) is instead directed to the protection 1×4 WRE 910′; (2) setting the 4×1 fiber switch 927 to IP1 and the 2×1 fiber switch 916(1) to IP1 so that traffic otherwise received from the first working 4×1 WRE 912(1) is instead received from the protection 4×1 WRE 912′; and (3) setting fiber switches 926, 928, 932, and 934 so that the O5β signals will be directed as would otherwise be the O1β signals and that the I5α′ signals will be received as would otherwise be the I1α′ signals. Specifically, (3) is achieved by: setting fiber switch 932(4) to direct O21 to I51; setting fiber switch 934(4) to direct O51 to I21; setting fiber switch 932(7) to direct O31 to I52; setting fiber switch 934(7) to direct O52 to I31; setting fiber switch 932(10) to direct O41 to I53; setting fiber switch 934(10) to direct O53 to I41; setting fiber switch 926(1) to direct O51 to I21; setting fiber switch 926(2) to direct O52 to I31; setting fiber switch 926(3) to direct O53 to I41; setting fiber switch 928(1) to direct O21 to I51; setting fiber switch 928(2) to direct O31 to I52; setting fiber switch 928(3) to direct O41 to I53; and leaving the settings of the other fiber switches 932 and 934 as they are for working traffic. Similar settings may be used in the event it is necessary to reroute traffic to respond to a failure in a different part of the working fabric 904.

[0096] While not shown explicitly in FIG. 9B, the distribute-and-select architecture with protection may also include amplifiers on the working and/or protection fabrics. Such amplifiers may be disposed as desired to compensate for loss of signal strength as optical signals are propagated through the system. In particular, such losses may be greater with the protection fabric as shown in FIG. 9B than those in FIG. 9A because of the power splitting introduced by the optical splitters 920. Also, while not explicitly shown, OPMs may additionally be included at various points in the cross connect to monitor the optical channels. Suitable positions include, among others: before the optical splitters 920 to identify signal faults before active operation of the cross connect 900′; after optical amplifiers positioned on the working and/or protection fabrics 904 and 908 to permit isolated signal monitoring of the WREs in diagnosing the source of detected faults; and after the 2×1 fiber switches 916 to permit identification of signal faults after active operation of the cross connect 900′. Additionally, OSC drops and adds may be provided respectively on the input and output signals 902 and 940 for removing and adding out-of-band signals.

[0097] It is evident that the basic architecture shown for a 4×4 cross connect in FIG. 9B may be extended to an arbitrary K×K cross connect that receives K input signals and emits K output signals and has F protection arrangements. The working fabric comprises a first set of K WREs each configured to operate as a 1×K WRE and a second set of K WREs each configured to operate as a K×1 WRE. The protection fabric comprises F similar pairs of 1×K WREs and K×1 WREs. A set of K optical splitters are disposed to be encountered by the input signals, each comprising a 1:(F+1) optical splitter and the K fiber switches transmitting the output signals comprise (F+1)×1 fiber switches. Each of the 1×2 fiber switches 932 is substituted with a 1×(F+1) fiber switch to redirect traffic to any of the protection arrangements and each of the 2×1 fiber switches 934 is substituted with an (F+1)×1 fiber switch to receive traffic from any of the protection arrangements. The fiber switches 926 and 928 associated with each protection arrangement are provided respectively as 1×K and K×1 fiber switches so that each protection arrangement may interchange traffic with any of the working components as necessary.

[0098] One advantage of the distribute-and-select architecture is that component failures are readily detected with a simple power detector placed on the output. This ability results from the fact that a failure can only act to remove a desired spectral band from one of the output signals, but cannot act to substitute the desired spectral band with an undesired spectral band. As an example, consider the failure of a single retroreflector in one of the WREs comprised by the working fabric. That failure will be manifested on the output signals as an absence of a desired spectral band. A further advantage to the distribute-and-select architecture is that the power splitting of the input signals is modest, even for very large cross connects, since they are used only to provide duplicate signals as needed for the protection fabric.

[0099] c. Cross-Connect Architectures Using Optical-Channel-Blocking Configurations

[0100] In some embodiments, the cross-connect architectures make use of optical-channel-blocking component WREs, which permit configurations in which spectral bands that appear at an input are blocked from propagation to an output of the component WRE. Specific examples of WRE arrangements that use component OChB WREs to effect higher-order WRE operations are shown in the third columns of FIGS. 6C, 6D, and 6E respectively for 2×1 OChB WREs, 3×1 OChB WREs, and 4×1 OChB WREs. In addition to the OChB WRE components, such arrangements may comprise one or more optical combiners. Inputs to the arrangement are provided to the plurality of component WREs, with outputs from each of the component WREs being combined by the one or more optical combiners. In some instances, it is convenient for all of the component WREs to accommodate the same number of inputs, i.e. for them all to be M×1 WREs, but this is not a requirement. For example, column 642(3) of FIG. 6C shows WRE arrangements that use only 2×1 WREs, column 644(3) of FIG. 6D shows WRE arrangements that use only 3×1 WREs, and column 646(3) shows WRE arrangements that use only 4×1 WREs. In other instances, the WRE arrangements may use combinations of WREs having different numbers of input ports. Merely by way of example, a 9×1 optical-channel-blocking WRE arrangement could comprise a 2×1 WRE, a 3×1 WRE, and a 4×1 WRE. The optical combiners are used to combine the outputs from each of the component WREs.

[0101] One embodiment of a cross-connect architecture that uses an OChB WRE is illustrated in FIG. 10. In this example, a 4×4 cross-connect architecture 1000 is illustrated, although it will be evident that the principles used may be extended to architectures of other sizes. As shown, the architecture permits routing of spectral bands from a plurality of input optical signals 1002 onto a plurality of output optical signals 1040, although it will be appreciated by those of skill in the art that the architecture 1000 may alternatively be used by propagating light in the opposite direction, thereby interchanging the roles of the input and output optical signals. The operation of the architecture 1000 is similar to the distribute-and-select architecture discussed above, but uses some of the channel-blocking features permitted by the use of the OChB WREs.

[0102] Thus, each of the input signals 1002 encounters a distribution arrangement 1004 configured to distribute their spectral bands among selection elements 1022. While the distribute-and-select architecture described above uses 1×K WREs for the distribution elements, the architecture 1000 shown in FIG. 10 uses a combination of smaller WREs and optical splitters to achieve a more limited form of distribution. For the 4×4 cross connect shown, each distribution element 1004 includes a 1×2 WRE 1006 with a pair of 1:2 optical splitters 1008. Spectral bands comprised by a particular input signal 1002(k) are thus distributed among two intermediate signals within the respective distribution element 1004(k), with those intermediate signals being duplicated by the 1:2 optical splitters 1008. The resulting signals are distributed among the selection elements 1022. One consequence of the duplication of intermediate signals by the optical splitters 1008 is that substantially identical intermediate signals may be provided to more than one of the selection elements 1022.

[0103] In one embodiment, the selection elements comprise optical-channel-blocking WRE arrangements that include at least one WRE with channel-blocking capability. As shown in FIG. 10, this may include a plurality of OChB WREs, with outputs of the plurality of such WREs being combined by an optical combiner. In the specific example of the 4×4 architecture 1000 shown in FIG. 10, each of the selection elements comprises a pair of 2×1 OChB WREs 1006′ with the outputs of the OChB WREs 1006′ combined by optical combiner 1008′. This structure corresponds to the configuration previously shown in the second row of column 642(3) of FIG. 6C. The inputs to the OChB WREs 1006′ are configured to receive signals output from the distribution elements 1004. For each selection element 1022, each input receives an intermediate signal from a different distribution element 1004. In one embodiment, the selection elements 1022 may be grouped with distinct subsets of the selection elements 1022 receiving equivalent sets of intermediate signals. This is the case, for example, in the specific embodiment shown in FIG. 10 in which selection elements 1022(1) and 1022(2) receive signals originating from the same set of optical splitters 1008; selection elements 1022(3) and 1022(4) similarly receive signals that originate from the same set of optical splitters.

[0104] The specific spectral bands comprised by the output signals 1040 are determined by configurations of the 1×2 WREs 1006 comprised by the distribution elements 1004 and the 2×1 WREs comprised by the selection elements 1022. These elements may be configured so that any spectral band from any of the input signals 1002 may be directed to at least a desired one of the output signals 1040. Similarly, when the architecture is used in direction reverse to that shown in FIG. 10, any spectral band from signals 1022 may be directed to a desired one of signals 1002.

[0105] The principles described with respect to the architecture 1000 shown in FIG. may also be used for cross connects of other sizes. For example, an arbitrary K×K cross connect may be configured to function with a duplication level d. In such an embodiment, each of the K input signals encounters a distribution element that outputs K intermediate signals in d groups, i.e. so that sets of K/d intermediate signals are equivalent. This may be achieved in some embodiments with a distribution element that comprises a 1×d WRE that receives the respective input signal and outputs each of d signals to a 1:K/d optical splitter. Each of the K intermediate signals output from the distribution element is provided as an input to a different one of K selection elements that each comprise a WRE with the capability of blocking on optical channel. Each of the selection elements receives an intermediate signal from each of the distribution elements and extracts the desired spectral bands for propagation as one of the output signals depending on a configuration of the selection element. One structure that permits such functionality for each of the selection elements comprises a number d of K/d×1 WREs that receive the K intermediate signals. Outputs of the K/d WREs are combined by a d:1 optical combiner. In some embodiments, the cross connect may be configured so that a group of selection elements receive equivalent signals. This may be accomplished by providing the equivalent intermediate signals from the distribution elements to corresponding selection elements, resulting in a grouping of the K distribution elements into K/d groups that each include d distribution elements. The 4×4 architecture 1000 shown in FIG. 10 corresponds to the case where K=4 and d=2.

[0106] In reverse operation, input optical signals are provided to a plurality K of 1×K OChB WREs. The spectral bands for each output optical signal are selected by a d×1 WRE that receives, at each of its d inputs, combinations of spectral bands from K/d of the 1×K OChB WREs.

[0107] In still other embodiments, these principles may also be used to construct a asymmetric cross connect. For a K₁'K₂ cross connect, a set of K₁ distribution elements may each output K₂ intermediate signals, such as by using an arrangement comprising a 1×d WRE in communication with a plurality d of 1:K₂/d optical splitters. The intermediate signals are received by K₂ selection elements, each selection element receiving K₁ intermediate signals and comprising a WRE with channel-blocking capability. The desired operation of the selection elements may be achieved in one embodiment with each selection element having a plurality d of K₁/d×1 WREs coupled with a d:1 optical combiner.

[0108] Another embodiment that makes use of WREs having optical-channel-blocking capability is shown in FIG. 11. Like the embodiment described with respect to FIG. 10, this embodiment uses two distinct sets of elements to achieve the routing functions. A specific illustration is provided for a 4×4 cross connect 1100, although the principles may readily be applied to cross connects of other sizes, as will be evident to those of skill in the art. The first set of elements 1104 may be considered to be types of distribution elements, although they function differently than the distribution elements described previously. Each of these elements 1 104 receives an input optical signal 1102 having a plurality of spectral bands and provides intermediate signals, at least some of which are equivalents to the input signal 1102. This may be achieved by using elements that comprise an optical splitter 1108 that passes equivalents to the input signal; one of the equivalents is passed to one of the second set of elements, perhaps after further splitting by another optical splitter 1108, and another of the equivalents is provided to an OChB WRE 1106.

[0109] While each of the first set of elements 1104 performs the same type of distribution of spectral bands, two different structures are used for the second set of elements, depending on the origin of the intermediate signals received by those elements. Elements 1122 function as 4:1 optical combiners and combine the spectral bands received from the OChB WREs 1106 components of each of the first set of elements 1104. The configurations of these OChB WREs 1106 thus determine which spectral bands are included on the output optical signals 1140(1) and 1140(2) associated with the 4:1 optical combiners 1122. The 4:1 optical combiners 1122 are shown as comprising a cascaded arrangement of 2:1 optical combiners 1108′, but may more generally include any arrangement that provides the 4:1 optical combination.

[0110] The signals that are propagated from the first set of elements 1104 as equivalents to the input signals 1102 are received by elements configured as 4×1 OChB WREs 1124. In one embodiment, the functionality of the 4×1 OChB WRE 1124 is achieved by using a pair of 2×1 OChB WREs 1106′ whose outputs are combined by a 1:2 optical combiner 1108′. This structure is the same as used for the selection elements 1022 in the embodiment of FIG. 10 and corresponds to the OChB WRE structure shown in the second row of column 642(3). These 4×1 OChB WREs 1124 thus receive as inputs equivalents to all of the input signals 1102. The configurations of the component 2×1 OChB WREs for a particular 4×1 OChB WRE 1124 thus determine which spectral bands from the input signals 1102 are propagated as part of the respective output signals 1140.

[0111] While the above description of the operation of the architecture 1100 has 20 looked at one direction for the propagation of light, it will be apparent to those of skill in the art that it may alternatively be used with light propagating in the opposite direction.

[0112] Regardless of which direction light propagates in the system, it is possible to direct a spectral band from any of the input (output) signals 1102 (1140) to any of the output (input) signals 1140 (1102).

[0113] Also, while the above description has provided an example of this architecture for a 4×4 cross connect, the same principles may be used for differently sized cross connects. In such a configuration, a symmetric K×K cross connect receives K input optical signals at a first set of K elements that each output K intermediate optical signals. A portion, such as half, of the intermediate optical signals output from each of the first set of elements are equivalent to the respective input optical signals and the remainder carry spectral-band subsets of the respective input optical signals, depending on a state of a 1×K/2 OChB WRE comprised by the element. In some instances, the 1×K/2 OChB WRE may itself comprise an arrangement of smaller component OChB WREs. Each of the intermediate optical signals that is equivalent to one of the input optical signals is received by a K×1 OChB WRE, and each such K×1 OChB WRE receives an equivalent to each of the input optical signals. The K×1 OChB WRE may comprise an arrangement of smaller OChB WREs. Each of the intermediate optical signals that carry spectral-band subsets of the input optical signals is received by a K:1 optical combiner, and each such K:1 combiner receives an intermediate optical signal having a spectral band from each of the input optical signals. The K:1 optical combiner may comprise an arrangement of smaller optical combiners or may comprise a larger optical combiner with some unused ports. The number of K×1 OChB WREs and K:1 combiners is thus equal to K/2.

[0114] In reverse operation, such an architecture receives half of the input optical signals at 1:K optical splitters and the remainder of the input optical signals at 1×K OChB WREs. The components of each output signal are selected by combining outputs from each of the 1×K OChB WREs with spectral bands selected by a 1×K/2 OChB WRE from equivalents to the optical signals provided to the 1:K optical splitters.

[0115] There is also no requirement that a cross connect using the architecture illustrated in FIG. 11 be symmetric. In instances where the principles are adopted for an asymmetric K₁×K₂ cross connect, each of the first set of elements receives one of K₁ input signals and outputs K₂/2 equivalents to the respective input signal as well as K₂/2 intermediate signals that include a subset of the spectral bands from the respective input signal. Each of the K₂/2 equivalents is received by one of K₂/2 elements that acts as a K₁×1 OChB WRE. Each of the other intermediate signals is received by one of K₂/2 elements that acts as a K₁:1 optical combiner. The output signals correspond collectively to the signals output by the K₁:1 optical combiners and the K₁×1 OChB WREs.

[0116] d. Other Cross-Connect Architectures

[0117] Other cross-connect architectures that use at least one WRE are also within the scope of the invention. A number of such alternative architectures are provided below as illustrations of specific embodiments, but it will be appreciated that these illustrations are not intended to be limiting and that there are various other alternative embodiments that are also within the scope of the invention.

[0118] 1. Crossbar Cross Connects

[0119]FIG. 12A shows an example of a 4×4 crossbar cross connect 1210. This 4×4 cross connect 1210 comprises a two-dimensional arrangement of 2×2 cross connects 1212, each of which may be configured according to the broadcast-and-select or distribute-and-select architectures described above. The input signals 1218 are received by respective ones of the 2×2 cross connects 1212 distributed along a first dimension of the array. The output signals 1220 are emitted by respective ones of the cross connects along the second dimension of the array.

[0120] Within the array, one of the two output ports for each 2×2 cross connect 1212 is optically connected with an input port of the next 2×2 cross connect 1212 in the first of the two dimensions and the second of the two output ports is optically connected with an input port of the next 2×2 cross connect 1212 in the second of the two dimensions. Each of the edge 2×2 cross connects 1212 at the opposite side of the array from where the output signals 1220 are emitted has one unused input port 1214 since there is no 2×2 cross connect 1212 to couple with that input port 1214 in the first dimension. Similarly, each of the edge 2×2 cross connects 1212 at the opposite side of the array from where the input signals 1218 are received has one unused output port 1216 since there is no 2×2 cross connect 1212 to couple with that output port 1216 in the second dimension.

[0121] It is apparent how a more general K×K crossbar cross connect architecture may be provided with a K×K array of 2×2 cross connects configured in the same fashion as FIG. 12A but extended even further in the two dimensions of the array. Except for the edge elements where optical signals are received or transmitted, or have corresponding unused ports, each 2×2 element in the array receives a signal from an element next to it in both directions in the array and transmits a signal to an element next to it in both directions. Such a generalized crossbar cross connect still functions to distribute a plurality of spectral bands on the K input optical signals among the K output optical signals as desired.

[0122] 2. Benes Cross Connect

[0123]FIG. 12B provides an illustration of a 4×4 Benes cross connect 1230 configured to distribute spectral bands from multiplexed input optical signals 1238 among output optical signals 1240 as desired according to a configuration of the component 2×2 cross connects 1232. In this embodiment, six 2×2 cross connects 1232 are provided and configured as illustrated. The Benes cross connect 1230 includes three pairs of 2×2 cross connects 1232. The first pair receives the four input signals 1238 at its four input ports and the third pair transmits the four output signals 1240 at its four output ports. Each of the intermediate pair of 2×2 cross connects 1232 receives at its input ports signals from each of the first pair of 2×2 cross connects 1232 and transmits at its output ports signals to each of the third pair of 2×2 cross connects 1232. It will be appreciated that the example of a 4×4 Benes cross connect is used for illustrative purposes only. More generally, the Benes configuration may be used to construct K×K cross connects from a plurality of smaller component cross connects, such as with 2×2 cross connects.

[0124] 3. Benes Cross Connect With Spatial Dilation

[0125]FIG. 12C provides an illustration of a 4×4 Benes cross connect with spatial dilation 1250. As for the other embodiments, the cross connect 1250 distributes spectral bands from multiplexed input optical signals 1262 among output optical signals 1264 as desired according to a configuration of the cross connect 1250. In this embodiment, two 4×4 cross connects 1256 are used in combination with four 2×1 WREs 1260. Each of the four input signals 1262 encounters an optical splitter 1252 to direct a duplicate of each input signal to each of the 4×4 cross connects 1256. Each of the 2×2 WREs 1260 is similarly configured to receive one of the outputs from each of the 4×4 cross connects 1256. The spatial dilation of the architecture has the effect of overcoming the rearrangably nonblocking character of the Benes architecture. It thus allows for a strictly nonblocking arrangement without requiring rearrangements when switching new connections. It will be appreciated that the example of a 4×4 Benes cross connect with spatial dilation is used for illustrative purposes only. More generally, the Benes configuration with spatial dilation may be used to construct K×K cross connects by increasing the capacity of the intermediate cross connects 1256.

[0126] 4. Clos Cross Connect

[0127]FIG. 12D provides an illustration of a 4×4 Clos cross connect 1270 that distributes spectral bands from multiplexed input optical signals 1278 among output optical signals 1279. The Clos cross connect 1270 comprises three layers of cross connects. The first layer comprises two 2×3 cross connects 1272 that receive the input signals 1278 at their four input ports. The third layer comprises two 3×2 cross connects 1276 that transmit the output signals 1279 from their four output ports. The 2×3 and 3×2 cross connects 1272 and 1276 may be configured in one embodiment as described above with respect to FIGS. 6A and 6B. The intermediate (second) layer comprises three 2×2 cross connects 1274, with each such 2×2 cross connect 1274 being disposed to receive signals at its input ports from both of the first-layer 2×3 cross connects 1272 and to transmit signals from its output ports to both of the third-layer 3×2 cross connects 1279. In certain configurations, the Clos cross connect 1270 effectively operates by using the first layer of 2×3 cross connects 1272 to distribute spectral bands among the 2×2 cross connects 1274 of the intermediate layer; from here, they are routed as desired to the 3×2 cross connects 1276 of the third layer for integration as desired onto the output signals 1279.

[0128] The basic structure of the Clos cross connect may be generalized as shown in FIG. 12E to act as a K×K cross connect 1280 for distributing a plurality of spectral bands on K input signals 1288 among K output signals 1289. As for the 4×4 Clos cross connect discussed with respect to FIG. 12D, the generalized Clos cross connect 1280 comprises three layers of elemental cross connects. The first layer comprises a plurality m of n×k cross connects 1282 that receive the K input signals 1288. In an embodiment where the number of input ports provided by the n×k cross connects is exactly equal to the number K of input signals, m≡K/n, although this is not a necessary requirement for an operation cross connect 1280. Similarly, the third layer comprises a plurality m of k×n elemental cross connects that transmit the K output signals 1289. The intermediate layer comprises a plurality k of m×m cross connects 1284 disposed so that each m×m cross connect 1284 receives signals from all of the first-layer n×k cross connects 1282 at its input ports and provides signals to all of the third-layer k×n cross connects 1286 from its output ports. For the generalized cross connect 1280 to operate without spectral bands blocking each other during their propagation through the system, k≧2n−1. Thus, in an embodiment where this nonblocking criterion is met exactly, k=2n−1.

[0129] A number of special cases of the Clos cross-connect configuration are explicitly noted. For example, the 4×4 Benes cross connect described with respect to FIG. 12B is equivalent to the configuration 1280 of FIG. 12E in the special case where n=m=k=2, and is therefore an example of a configuration in which the nonblocking criterion is not met exactly. The 4×4 Clos cross connect described with respect to FIG. 12D corresponds to nearly the same circumstances, but meets the nonblocking criterion exactly by having n=m=2 and k=3. Examples of larger cross connects that meet the nonblocking criterion exactly include an 8×8 cross connect in which n=2, k=3, and m=4, and a 16×16 cross connect in which n=2, k=3, and m=8. Still other examples of cross connects that may be formed are evident from the description above.

[0130] Having described several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. In particular, a number of examples have been provided illustrating discrete principles. Alternative embodiments may include elements in arrangements that use multiple of those principles. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims. 

What is claimed is:
 1. A method for distributing a plurality of spectral bands comprised by a plurality of input optical signals onto a plurality of output optical signals, the method comprising: selectively distributing spectral bands from each of the plurality of input optical signals onto a plurality of intermediate optical signals; duplicating the intermediate optical signals; and selecting spectral bands from combinations of the duplicated optical signals to provide the output optical signals.
 2. The method recited in claim 1 wherein selecting spectral bands from combinations of the duplicated optical signals comprises blocking propagation of certain spectral bands from the duplicated optical signals onto the output optical signals.
 3. The method recited in claim 1 wherein the combinations include spectral bands from each of the input optical signals.
 4. The method recited in claim 1 wherein the input optical signals are equal in number to the output optical signals.
 5. A method for distributing a plurality of spectral bands comprised by a plurality of input optical signals onto a plurality of output optical signals, the method comprising: selectively distributing spectral bands from each of the plurality of input optical signals onto a plurality of intermediate optical signals, wherein selectively distributing spectral bands comprises blocking propagation of certain spectral bands; and selecting spectral bands from combinations of the intermediate optical signals to provide the output optical signals.
 6. The method recited in claim 5 wherein the combinations include spectral bands from each of the input optical signals.
 7. The method recited in claim 5 wherein the input optical signals are equal in number to the output optical signals.
 8. A method for distributing a plurality of spectral bands comprised by a plurality of input optical signals onto a plurality of output optical signals, the method comprising: producing a first plurality of intermediate optical signals that are equivalent to corresponding input optical signals; producing a second plurality of intermediate optical signals by selectively distributing spectral bands from each of the plurality of input optical signals; combining combinations of the second plurality of intermediate optical signals to provide a portion of the output optical signals; and selecting spectral bands from each of the first plurality of intermediate optical signals to provide a remainder of the output optical signals.
 9. The method recited in claim 8 wherein selecting spectral bands from each of the first plurality of intermediate optical signals comprises blocking propagation of certain spectral bands from the duplicated optical signals onto the remainder of the output optical signals.
 10. The method recited in claim 8 wherein producing the second plurality of intermediate optical signals by selectively distributing spectral bands from each of the plurality of input optical signals comprises blocking propagation of certain spectral bands onto the second plurality of intermediate optical signals.
 11. The method recited in claim 8 further comprising optically splitting each of the input optical signals to provide first and second internal optical signals used respectively for producing the first and second pluralities of intermediate optical signals.
 12. The method recited in claim 8 wherein the input optical signals are equal in number to the output optical signals.
 13. The method recited in claim 8 wherein the first and second pluralities of intermediate optical signals are equal in number.
 14. A method for distributing a plurality of spectral bands comprised by a plurality of input optical signals onto a plurality of output optical signals, the method comprising: producing a first plurality of intermediate optical signals equivalent to a portion of the plurality of input optical signals; producing a second plurality of intermediate optical signals by selectively distributing spectral bands from each of a remainder of the plurality of input optical signals; and combining combinations of the second plurality of intermediate optical signals with selected spectral bands from the first plurality of intermediate optical signals to produce the output optical signals.
 15. The method recited in claim 14 wherein producing the first plurality of intermediate optical signals comprises optically splitting each of the portion of the plurality of input signals.
 16. The method recited in claim 14 wherein producing the second plurality of intermediate optical signals by selectively distributing spectral bands from each of the remainder of the plurality of input optical signals comprises blocking propagation of certain spectral bands onto the second plurality of intermediate optical signals.
 17. The method recited in claim 14 wherein the selected spectral bands from the first plurality of intermediate optical signals are obtained by selecting spectral bands from combinations of the first plurality of intermediate optical signals.
 18. The method recited in claim 17 wherein selecting spectral bands from combinations of the first plurality of intermediate optical signals comprises blocking propagation of certain spectral bands from the first plurality of intermediate optical signals.
 19. The method recited in claim 17 wherein the input optical signals are equal in number to the output optical signals.
 20. The method recited in claim 17 wherein the portion and remainder of the plurality of input optical signals are equal in number.
 21. An optical wavelength cross connect for receiving a plurality of input optical signals each having a plurality of spectral bands and transmitting a plurality of output optical signals each having one or more of the spectral bands, the optical wavelength cross connect comprising: a first plurality of elements, wherein: each of the first plurality of elements is disposed to receive one of the plurality of input optical signals; and each of the first plurality of elements comprises a first wavelength routing element adapted for selectively routing wavelength components between a respective first optical signal and a respective plurality of second optical signals according to a configurable state of such first wavelength routing element, wherein a mapping of the spectral bands comprised by the plurality of input optical signals to the plurality of output optical signals is determined by states of the first wavelength routing elements, the state of at least one of the first wavelength routing elements resulting in blocking of transmission of at least one of the spectral bands between the respective first optical signal and the respective plurality of second optical signals.
 22. The optical wavelength cross connect recited in claim 21 wherein the state of each of the wavelength routing elements comprised by the first plurality of elements results in blocking of transmission of at least one of the spectral bands between the respective first optical signal and the respective plurality of second optical signals.
 23. The optical wavelength cross connect recited in claim 21 wherein each of the first plurality of elements further comprises an optical splitter for duplicating one of the respective plurality of second optical signals.
 24. The optical wavelength cross connect recited in claim 21 wherein each of the first plurality of elements further comprises a second wavelength routing element adapted for selectively routing wavelength components between a respective first optical signal and a respective plurality of second optical signals according to a configurable state of such second wavelength routing element, wherein the mapping of the spectral bands is further determined by states of the second wavelength routing elements.
 25. The optical wavelength cross connect recited in claim 24 wherein each of the first plurality of elements further comprises an optical combiner for combining the respective first optical signals for the first and second wavelength routing elements.
 26. The optical wavelength cross connect recited in claim 21 further comprising a second plurality of elements, wherein: each of the second plurality of elements is disposed to receive an intermediate optical signal from each of the first plurality of elements; and each of the second plurality of elements comprises a second wavelength routing element adapted for selectively routing wavelength components between a respective first optical signal and a respective plurality of second optical signals according to a configurable state of such second wavelength routing element, wherein the mapping of the spectral bands is further determined by states of the second wavelength routing elements.
 27. The optical wavelength cross connect recited in claim 26 wherein each of the intermediate optical signals is equivalent to another of the intermediate optical signals.
 28. The optical wavelength cross connect recited in claim 26 wherein: the first wavelength routing element comprised by each of the first plurality of elements is adapted for selectively routing wavelength components between one of the input optical signals and d internal optical signals; and each of the first plurality of elements further comprises optical splitters for providing equivalents of the internal optical signals as intermediate optical signals, whereby the intermediate optical signals form d groups of equivalent optical signals.
 29. The optical wavelength cross connect recited in claim 28 wherein a plurality of the second elements receive equivalent sets of intermediate optical signals.
 30. The cross connect recited in claim 26 wherein each of the second plurality of elements comprises: a plurality of second wavelength routing elements; and an optical combiner disposed to combine the respective first optical signals from each of the second wavelength elements.
 31. The optical wavelength cross connect recited in claim 26 wherein each of the first plurality of elements comprises: a plurality of first wavelength routing elements; and an optical splitter disposed to split the input optical signal into a plurality of internal signals that correspond to the respective first optical signals for the plurality of first wavelength routing elements.
 32. The optical wavelength cross connect recited in claim 31 wherein each of the second plurality of elements further comprises a plurality of optical combiners disposed to combine the intermediate signals received by such each of the second plurality of elements to produce the respective second optical signal for the second wavelength routing element.
 33. The optical wavelength cross connect recited in claim 21 further comprising a second plurality of elements, wherein: each of the second plurality of elements is disposed to receive an intermediate optical signal from each of the first plurality of elements; each of a portion of the second plurality of elements comprises an optical combiner disposed to combine the intermediate optical signals received by such each of the portion of the second plurality of elements to produce a respective one of the output optical signals; and each of a remainder of the second plurality of elements comprises a second wavelength routing element adapted for selectively routing wavelength components between a respective first optical signal and a respective plurality of second optical signals according to a configurable state of such second wavelength routing element, wherein the mapping of the spectral bands is further determined by states of the second wavelength routing elements.
 34. The optical wavelength cross connect recited in claim 33 wherein a plurality of the intermediate optical signals are equivalents to the input optical signals.
 35. The optical wavelength cross connect recited in claim 34 wherein: intermediate optical signals that are equivalents to the input optical signals are received from the first plurality of elements by the remainder of the second plurality of elements; and intermediate optical signals that are not equivalents to the input optical signals are received from the first plurality of elements by the portion of the second plurality of elements.
 36. The optical wavelength cross connect recited in claim 33 wherein each of the first plurality of elements further comprises an optical splitter disposed to split the input optical signal received by such each of the first plurality of elements and to provide an equivalent to such input optical signal that corresponds to the respective first optical signal for the first wavelength routing element.
 37. An optical wavelength cross connect for receiving a plurality of input optical signals each having a plurality of spectral bands and transmitting a plurality of output optical signals each having one or more of the spectral bands, the optical wavelength cross connect comprising: a first plurality of elements configured to receive the input optical signals and to provide combinations of the spectral bands on a plurality of intermediate optical signals; and a second plurality of elements configured to receive the intermediate optical signals and to provide the output optical signals, wherein each of the second plurality of elements comprises a first wavelength routing element for selectively routing wavelength components between a respective first optical signal and a respective plurality of second optical signals according to a configurable state of such first wavelength routing element, wherein a mapping of the spectral bands from the input optical signals to the output optical signals is determined by states of the first wavelength routing elements, the state of at least one of the first wavelength routing elements resulting in blocking transmission of at least one of the spectral bands between the respective first optical signal and the respective plurality of second optical signals.
 38. The optical wavelength cross connect recited in claim 37 wherein each of the intermediate optical signals is equivalent to another of the intermediate optical signals.
 39. The optical wavelength cross connect recited in claim 37 wherein the state of each of the wavelength routing elements comprised by the second plurality of elements results in blocking of transmission of at least one of the spectral bands between the respective first optical signal and the respective plurality of second optical signals.
 40. The optical wavelength cross connect recited in claim 37 wherein each of the first elements comprises a second wavelength routing element adapted for selectively routing wavelength components between a respective first optical signal and a respective plurality of second optical signals according to a configurable state of such second wavelength routing element; and a plurality of optical splitters. 